Field of the Invention
The present invention relates to a tool path curve generation method and a tool path curve generation apparatus that ensure a shape error of a predetermined value or less.
Description of the Related Art
There have been widely known methods for determining a curve that interpolates a given point sequence or a curve that passes by each point of a point sequence (Les Piegl, Wayne Tiller, The NURBS Book, Springer-Verlag, pp 405-453, and I. J. Schoenberg, Spline functions and the problem of graduation, Proceedings of the National Academy of Sciences of the U.S.A., 52 (1964), pp 947-950). In a machining process by a numerically controlled machine tool, these methods are used for converting a tool path formed of a point sequence into a curve in order to achieve a smoother machined surface and reduced machining time (Japanese Patent Application Laid-Open No. 2013-171376, Japanese Patent Application Laid-Open No. 2007-293478, Japanese Patent Application Laid-Open No. 2006-309645, Japanese Patent Application Laid-Open No. 2005-182437, Japanese Patent Application Laid-Open No. 2004-078516, and Japanese Patent Application Laid-Open No. H10-240329).
In general, the length of a machining program varies depending on a workpiece. Time-consuming machining involves an extremely large number of points constituting a tool path, and therefore, it is difficult to complete the conversion of the tool path into a curve in one process. For this reason, a method has been used in which a point sequence is divided up to generate curves in small increments. This method is inevitably used to generate curves and perform machining in parallel.
The curves generated from divided groups of points are expressed in the form of parametric curves. An expression form, such as a B-spline curve or a NURBS curve, which is suited for each application, is adopted.
In creating the curves of point sequences described above, efforts were initially focused on the generation of curves (interpolation curves) that pass the points of a point sequence. However, the recent tendency is toward creating curves that pass by a point sequence (approximated curve) rather than curves that pass a point sequence, mainly taking into account a shape error that occurs when creating a machining program by CAM. An approximated curve is generally conditioned that each point of a point sequence lies within a predetermined distance from the curve (Japanese Patent Application Laid-Open No. 2013-171376).
While a curve generated from a point sequence provides a smooth path, the curve is required to pass by an original point sequence. For this reason, according to a conventional method for creating curves, a curve is generated such that the distance between each point of a point sequence and a point corresponding thereto on the curve will take a preset value or less.
However, according to the foregoing method, the attention is focused on the distance from a curve only at a command point, so that the curve may considerably deviate from the point sequence at other places. This has been posing a problem in that the method cannot be applied to a case where the shape error of a tool path resulting from the generation of curves is desirably controlled to a predetermined value or less.
Further, Japanese Patent Application Laid-Open No. H10-240329 describes a method in which, in the case of an interpolation curve, some points are taken on a curve sandwiched between command points, and the distance between a segment that connects the command points and each of the points is observed. This method allows more detailed determination than the method in which the distance is observed only at command points. However, the section sandwiched between adjacent points is not checked, thus failing in principle to ensure that an error is an allowable value or less.